Multiple neuromodulators are known to exist within the cell bodies of the retina's inner nuclear layer; some of these have been shown to be released by flashes of light. The function of these modulators following their release is currently unknown. To study this, we have altered the release of these substances pharmacologically with either antagonists or agonists. Baseline electroretinograms (ERG) were compared with those influenced by the administered drug. The effects of several drugs were studied: (1) Patients with Parkinson's disease were taken off all medication for 24-48 hours as part of another protocol; and baseline ERGs were then obtained. A second ERG was obtained following 2 hours of continuous L-Dopa infusion. Comparison of these ERGs suggest that application of exogenous dopamine in these patients increases the amplitude of the rod and cone b wave. (2) Other patients with Hemi-Parkinson's disease (and never on dopaminergic drugs) had their ERGs evaluated bilaterally: There were no obvious differences between the eye ipsilteral to the Parkinsonian tremors and the contralateral eye. (3) Patients with schizophrenia had placebo in place of haloperidol as part of another protocol; these patients remained "drug free" for at least two weeks. By comparison to age and sex matched norms, the implicit time of the "blue" cone b-wave was increased. This was confirmed in a second, independent study. (4) In normals, the specific dopamine antagonist metoclopramide was evaluated following an IV bolus administration. Cone a wave and b wave amplitudes were reduced under both dark and light adapted conditions. This reduction was more pronounced under dark-adapted conditions, implying that the neuro-modulatory function of dopamine is more marked in the dark.